1. Field of the Invention
The invention relates generally to seal assemblies for sealing between a rotating and a static member. In one aspect, and more particularly, the invention relates to seals for rolling cone bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. Still more particularly, the invention relates to dynamic seals of multi-material construction that are employed to protect the bearing surfaces between the rolling cone cutters and the journal shafts on which they rotate.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A typical earth-boring bit includes one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the drillstring and bit are rotated, the cone cutters thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts (also called journal shafts or pins) that extend downwardly and inwardly from the journal segment legs. As the bit is rotated in the borehole, each cone cutter is caused to rotate on its respective journal shaft as the cone contacts the bottom of the borehole. The borehole is formed as the action of the cone cutters removes chips of formation material (“cuttings” or “drilled solids”) which are carried upward and out of the borehole by the flow of drilling fluid which is pumped downwardly through the drill pipe and out of the bit. Seals are provided in glands between the rolling cones and their shaft to prevent lubricant from escaping from around the bearing surfaces and to prevent the cutting-laden, abrasive drilling fluid from entering between the cone and the shaft and damaging to the bearing surfaces. Liquid drilling fluid is normally used for oil and gas well drilling, whereas compressed air is generally used as the drilling fluid in mining operations. When cuttings are conveyed into the seal gland, they tend to adhere to the gland and/or seal component surfaces and may cause deformation, damage and/or slippage of the seal components. Moreover, the cuttings can accelerate abrasive wear of all seal components and of the bearing surfaces.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string in order to replace the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. It is therefore advantageous to maximize the service life of a drill bit in a rock formation. Accordingly, it is always desirable to employ drill bits that will be durable enough to drill for a substantial period of time with acceptable rate of penetration (ROP).
The durability of a bit and the length of time that a drill bit may be employed before it must be changed depends upon numerous factors. Importantly, the seals must function for substantial periods under extremely harsh downhole conditions. The type and effectiveness of the seals greatly impact bit life and thus, are critical to the success of a particular bit design.
One cause of bit failure arises from the severe wear or damage that may occur to the bearings on which the cone cutters are mounted. These bearings can be friction bearings (also referred to as journal bearings) or roller type bearings, and are typically subjected to high drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures.
As previously mentioned, the bearing surfaces in typical bits are lubricated, and the lubricant is retained within the bit by one or more seals. The seal is typically in the form of a ring and includes a dynamic seal surface that is placed in rotating contact against another surface and a static seal surface that is placed in contact against a stationary surface. Although the bit will experience severe and changing loading, as well as a wide range of different temperature and pressure conditions, the dynamic and static seal surfaces must nevertheless remain sealingly engaged in order to prevent the lubricant from escaping and/or contaminants from entering the lubricated areas, and should perform these duties throughout the life of the bit's cutting structure.
A variety of seal types are known in the art. These include O-ring type seals made entirely from a single type of rubber or other elastomeric material. The service life of bits equipped with such elastomeric seals is generally limited by the ability of the elastomeric seal material to withstand the different temperature and pressure conditions at each dynamic and static seal surface.
Another example of a conventional O-ring seal is one having a dynamic seal surface formed from a first type of elastomeric material, and a static seal surface that is formed from an elastomeric material different than that used to form the dynamic seal surface. The elastomeric material used to form the static seal surface may be less wear resistant than the material used to form the dynamic seal surface. Although such seal construction provides improved wear resistance at the dynamic seal surface when compared to single-elastomer seals, the amount of wear resistance and seal life that is provided is still limited by the properties inherent in the elastomeric materials.
More recently, certain metal-to-metal seals have been employed in certain rolling cone bits. Such metal-to-metal seals were developed in order to increase the working life of the bearings given that the failure of conventional elastomeric O-rings was one of the most frequent causes of bit failure, such failure manifesting itself in a loss or breakage of inserts on the rolling cone cutters, and even loss of entire cones. However, as compared to a conventional, elastomeric o-ring seal, the metal-to-metal seal assemblies have required substantially greater axial space along the journal surfaces in order position and retain the various components that make up the seal. In part, this increased axial space was required because such seal assemblies not only required the metal components that engage one another in providing the seal, but in addition required one or more spring-like “energizers” to cause the metal sealing surfaces to remain in engagement with one another. Some such energizers, to be effective, had to be relatively large. The space to retain these multiple-component assemblies is substantially greater than that necessary to house a single, elastomeric o-ring, for example. Relatively large diameter bits that included correspondingly large cone cutters could accommodate such multiple-component, metal-to-metal seal assemblies; however, in smaller sized bits, such as 8.5 inches in diameter and below, which account for a very large portion of bits made and employed, such space was hardly available, meaning that the bit designer was forced to accept design compromises. For example, the designer could employ the use of an elastomeric, o-ring seal with its relatively shorter life, or use a metal-to-metal seal with smaller than desired energizers due to the space limitations, or he could provide the space in the cone required for the desired metal-to-metal seal assembly, but at the expense of cone and leg strength due to the removal of additional cone steel and the relocation of bearings as required to house and retain the components.
Further, with respect to metal-to-metal seals, great care and attention must be employed in their manufacture to ensure that the engaging metal surfaces are extremely flat so as to ensure a good seal. Manufacturing such seals is time consuming and expensive. In many conventional designs, a lack of flatness in the engaging surfaces, misalignment of the engaging surfaces upon assembly or later use, and similar irregularities can be detrimental to seal life and thus to bit life.
It is therefore desirable that a new, durable and long lasting seal assembly be devised, one having the benefits offered by metal-to-metal seals, including long life and relative insensitivity to high temperatures, pressures and extreme forces, but not requiring such an axial length that either the seals cannot be properly energized, or that necessitates a removal of cone steel such that the cone or bearing surfaces are detrimentally weakened. Preferably, such seal assemblies would be constructed in a manner that would provide longer life than seals formed exclusively from elastomeric materials, and provide the enhanced life and wear resistance desired of metal-to-metal seals. Further still, it would be preferred that the components of the seal assembly yield manufacturing efficiencies and provide an ability to self-adapt and thereby maintain sealing engagement between the seal components despite the fact that the cone and seals will experience rapid changes in harsh drilling conditions.
Accordingly, to provide a drill bit with better performance and longer life, and thus to lower the drilling costs incurred in the recovery of oil and other valuable resources, it would be desirable to provide a seal that has the potential to provide longer life than conventional elastomeric seals, and at the same time, does not require as much axial space along the journal surfaces as compared to many conventional metal-to-metal seal assemblies. Preferably, such seals would provide a bit that will drill with acceptable ROP for longer periods than bits employing conventional seals so as to increase bit life and increase in footage drilled.